In block polymerization, there is substantially effected a combination of the best physical and chemical properties of two or more polymers, for example, the combination of those of polypropylene with those of polyethylene. Thus, polyethylene, while not possessing melting points or tensile strengths as high as those of propylene, does in fact possess excellent low temperature properties such as brittleness and impact. When the outstanding properties of both of these polymers are combined in the technique of block polymerization, there results at once a heteropolymer useful in many applications for which neither homopolymer was practically useful.
A group of block copolymers, which have excellent physical properties, are the ethylene-propylene block copolymers, e.g. those of the type P-EP, where P denotes a propylene homopolymer preblock and EP is a post-block of ethylene-propylene copolymer. By varying the proportions of the blocks and the polymerized ethylene content, the physical properties can be closely controlled to fit the particular application for which the polymer products are intended. In general, at constant melt flow rates the impact strength at room temperature of the block copolymer is substantially directly proportional to the amount of polymerized ethylene in the total product.
Block copolymers are advantageously produced on a commercial scale by the process disclosed in U.S. Pat. No. 3,514,501. Briefly, this process involves preparation of the preblock, preferably in the liquid phase, by catalytic polymerization of propylene in a hydrocarbon diluent such as liquid propylene to form a slurry. After separation of the slurry, the prepolymer which still contains active catalyst residues is introduced into at least one reaction zone, where it is reacted with monomer vapors for a sufficient period of time to form the polymer post block onto the polymer preblock in the desired proportions.
In the past the conventional catalyst system used in such a polymerization process has been an unmodified or an electron donor-modified titanium halide component, activated with an organo-aluminum cocatalyst. Typical examples of conventional propylene polymerization catalyst systems include cocrystallized titanium trichloride-aluminum trichloride catalysts of the general formula n.TiCl.sub.3.AlCl.sub.3 activated with diethyl aluminum chloride or triethylaluminum. The cocrystallized titanium trichloride-aluminum trichloride can have been subjected to a modification treatment with a suitable electron donor compound to increase its activity or stereospecificity. Such compounds include phosphorus compounds, esters of inorganic and organic acid ethers and numerous other compounds.
One major drawback, however, in using the aforementioned conventional catalysts, has been the low catalyst productivity, which has necessitated the subsequent deashing of the product to reduce the content of catalyst residues, which otherwise would detrimentally affect the product quality.
Recently new catalysts have been developed which are far more active than the aforementioned conventional catalysts in the polymerization of .alpha.-olefins. Briefly described, these catalysts are comprised of a titanium halide catalyst component supported on magnesium dihalide and an alkylaluminum compound, which can be present as a complex with an electron donor compound. These catalyst compounds have been described in the patent literature, e.g. in U.S. Pat. Nos. 3,830,787, U.S. Pat. No. 3,953,414, U.S. Pat. No. 4,051,313, U.S. Pat. No. 4,115,319.
The productivities obtained with these new catalyst are extremely high resulting in polymers containing such small quantities of residual catalyst that the conventional deashing step can be dispensed with. The catalysts function well in the homopolymerization of propylene and in the copolymerization of a mixture of propylene and another .alpha.-olefin such as ethylene, provided that the polymerization reaction is carried out in a liquid diluent, e.g. liquid propylene monomer. However, in the vapor phase polymerization used in preparing the EP copolymer block of the P-EP block copolymer described above, using conventional operating conditions, it has been found that the product quality of the resulting block polymer has been substantially inferior. Specifically, in order to achieve a desired impact strength at a desired melt flow, it was found that considerably more ethylene had to be incorporated into the total polymer than is the case when employing conventional catalyst. The necessary increase in ethylene content to achieve the impact strength detrimentally affects other desirable properties of the final product such as stiffness, heat deflection temperature, tensile properties, etc.
It is therefore an object of the present invention to provide a highly efficient process for the vapor phase polymerization of ethylene-propylene blocks onto a preformed propylene polymer yielding polymer products having improved impact strength without significantly affecting other desirable physical polymer properties.
Another object of the invention is to provide a process for the preparation of ethylene-propylene block copolymers wherein the polymerized ethylene content of the total polymer product is minimized to achieve a desired impact strength.
Further objects will become apparent from a reading of the specification and claims.